The ability of a, very thin and very narrow, section or “nanowire” of superconducting material to detect the impact of a photon has long been known. The basic principle is as follows. A nanowire of a superconducting material is created and electrically wired to a voltage source. As the current flows through the wire it creates heat. If the cooling needed to reach the temperature for superconducting phenomena (TC of approximately 10 kelvin or less) and the heating due to the current density in the nanowire are properly balanced, then the nanowire can be held extremely close to, but under, the superconducting phase transition. Below this critical temperature, TC, the wire effectively has no resistance. Just above the critical temperature the wire is not superconducting and has a finite resistance. When a photon strikes the superconducting nanowire it breaks the cooper pairs in the vicinity and creates a hot spot. If this photon induced hotspot raises the temperature of the segment of nanowire above TC then the hotspot will undergo a phase transition and no longer be superconducting. If the non-superconducting area is large enough, or equivalently the nanowire is small enough, it will block the entire nanowire. This leads to a sudden rise in the resistance of the nanowire. This change in resistance can be detected by various electronic setups and a single photon is counted. Once the hot spot from the incident photon disperses, the wire will return to the superconducting state and the device will be ready to detect another photon. This is called the reset time of the device.
Traditional Superconducting Nanowire Single Photon Detectors (SNSPDs) are made from one long nanowire. In order for this single nanowire to cover a useful area a “meander” is created. In effect the wire is folded back and forth across the surface of the desired area, usually about 10 μm by 10 μm (microns). These devices are called “single photon detectors” because the nanowire can only feel the loss of the superconducting condition somewhere along its length. If two photons hit the nanowire at the same time, two hot spots are created but only the increase in resistance is felt so the detector can “see” only one photon. Such detectors are effectively high pass filters; they can detect the presence of 1 or more photons without being able to count them. Similarly the device has no means of measuring spatial resolution. The output signal of the device is not a function of the location of the photon impact.
A second drawback of the long nanowire approach is the relaxation time of the detector. It has been shown that the relaxation time, the time for a hot spot to dissipate, is related to the kinetic-inductance and thus the length of the nanowire. This leads to a relaxation time of about 10 ns for a niobium nitrite, NbN, single wire 10 μm by 10 μm meander. The operational repetition rate will need to be significantly slower than the relaxation time of the device to avoid interactions between the relaxation and incoming photons. This leads to experimental repetition rates much slower than current pulsed laser systems which are capable of gigahertz frequencies.
To create the number resolution of the overall optical device multiple nanowires detectors are needed. In U.S. Patent Application Publication No. 2009/0050790A1 by Dauler et al., one possible method to gain some amount of number resolution is given, the so called multi element superconducting nanowire single photon detector (MESNSPD). Their method involves interleaving multiple long nanowires in a parallel meander across the surface of a substrate chip. While this solution is somewhat effective in creating number resolution, it has numerous drawbacks common to all current SNSPDs and the limited number of nanowires available, currently 10 or so, limits the number resolution of the device.
One of the most important effects in a SNSPD is that of current crowding. Current crowding in superconducting nanowires has been studied by Clem and Berggren. The heating in the superconducting nanowire, as mentioned above, is vital and is determined by the local current density along the wire. Ergo an area with higher current density will be “hotter” than an area of low current density. In order to maintain the necessary superconducting condition along the full length of the wire the maximum bias current applied through any nanowire detector is determined by the point of highest current density and therefore highest temperature. Current crowding as discussed by Clem and Breggen describes the effect of bends and constrictions in the nanowire which increase the local current density, such as the bends in the multi element superconducting nanowire single photon detector (MESNSPD) of Dauler et al. and standard SNSPDs. These bends are then the hottest spots on the detector, i.e. the closest to the critical temperature TC. This is a significant problem as the quantum efficiency of any region of a nanowire is proportional to how close that nanowire region is to TC. The incident single photon carries a very small amount of energy and creates a small amount of heating. The closer the operating temperature, sometimes called bias temperature TB, is to the TC the higher the efficiency of that region of nanowire is. The quantum efficiency of the device can then be thought of as the average efficiency of all the segments of the wire. If one segment is higher in temperature, such as the bends in a current SNSPD or MESNSPD, then ALL of the rest of the wire will have a lower efficiency. We also note that the high efficiency corners are often left out of what is considered the “active” area of many devices.
It should also be noted that in all current SNSPDs or MESNSPDs the current crowding effect limits the fill factor. Fill factor is the ratio of the active area that can detect a photon to that which can't, i.e. the ratio of the nanowire area to substrate area within the active detection area. Yang et. al teach in their article “Suppressed Critical Currents in Superconducting Nanowire Single-Photon Detectors with High Fill-Factors” that, as the name implies, large fill factor detectors have lower efficiency. This is a counterintuitive result as one would expect that the more photon sensitive area there was within the active area the higher the efficiency would be. In the work of Clem and Breggen it is noted that the sharper the bend in a nanowire the worse the effect of current crowding becomes. As the fill factors increase in all single layer SNSPDs and MESNSPDs the bends approach the worst case of sudden 180 degree turns. This limits the fill factor of any traditional detector and thus reduces the total quantum efficiency.
Another drawback of SNSPDs that likewise persists in MESNSPDs are long reset times. The reset time of a SNSPD is directly proportional to the kinetic inductance of the nanowire and can be altered by altering the geometry of the nanowire, such as its thickness or width. For a given geometry, the kinetic inductance is directly proportional to the length of the nanowire. Thus the reset time can be adjusted by changing the length of the superconducting nanowire, that is shortening the wire will provide the benefit of a shorter reset time. This is a significant drawback to existing SNSPND and MESNSPD. Because they are based on long continuous wires the effect of reducing the length of the wire is to reduce the size of the active area of the detector. Such a reduction in area is often unacceptable as the input photons are difficult to focus on a small area. In the MESNSPD this can be countered to a degree by adding more wires into the meander pattern, thus spreading it back outward, but even here the nanowires remain tens of microns long.
One definition of an ideal number resolving photon counter would be an analog of modern Charged Coupled Device (CCD) cameras. In that example, it is desired to have a large two dimensional array of pixels that covers the active area with the maximum number of small pixels, which are placed as compactly as physically possible. This would lead to being able to resolve large numbers of photons (from the large number of pixels) while having the shortest practical reset time and good spatial resolution (both from the small pixels), and the highest fill factor (from the closely packed pixels).
A final comment that must be mentioned with regard to the prior art is the lack of spatial resolution. The SNSPDs and MESNSPDs discussed above have little to no spatial resolution because each nanowire crosses the full, or at least a significant percent of in the case of non interleaved MESNSPDs, active area of the device.
What is needed therefore and has to date not been provided by the prior art is a superconducting nanowire photon detector that allows for improvements across a wide area of device characteristics, such as a reset time, fill factor, high quantum efficiency, spatial resolution and most importantly highly efficient resolution of the number of incident photons (number counting). This is needed as many experiments and applications in quantum optics require number resolution to be able to produce the correct results. The recently developed MLSNPD of Smith (U.S. patent application Ser. No. 13/506,857) addresses and improves upon many of these areas through the use of a novel Multi-layer architecture that placed the interconnected leads and detection elements on different layers of the device that were insulated against cross talk. While this more complicated architecture is effective it is desirable to simplify such a design.